Most times the sand will harden like you are describing from new tanks being dosed with too much Calcium and Alkalinity.
With new sand beds and new rock the tank chemistry can be a bit whacky seemingly eating up the carbonates put into the water.
This more times than none is due to over dosing the two products, one or the other and it precipitates into the sand creating rather concrete formation.
Noticing the sand getting hard can be hard at times if you do not stir it or check for this in new tanks and once it starts its hard to stop it relatively speaking as the hardened sand or other areas of precipitation will attract more of the carbonates to keep growing. Just the way this works.
In my last tank transfer back to a 6 foot tank I used all my well seasoned live rock but all new sand , fiji Pink sand.
I was dosing immediately due to the amount of corals in the system from transfer.
Slightly adjusting for the now larger water volume I still managed to get lop sided and to the point of dosing too much as the sand was taking up a bit more than I realized. Process started and kept going.
Large water change and stopped dosing until I got the dropping values recorded and resumed dosing at slightly lower level.
After a couple of months and breaking the sand up in tank back to actual sand OR removing it to add some vinegar / water to a bucket and allowing it to dissolve the ''cement'' then adding it back to the tank after a rinse the tank hit a point of equilibrium along with the new sand now well on its way to getting established and issue went away.
I did however have some spots on the bottom of my acrylic sump that is white where sheets of precipitation was hard to see had formed and once I removed it, No more issue there either.
New tanks are just that, new. Even transferring tanks that are well established and stable creates instabilities once again. New sand is just the same in a brand new tank or in an established one, Just one more reason to take things slow and allow the system to chill while its becoming an ecosystem and not start collecting corals and dosing products immediately. But we all have impatience issue to some form or another.
Plus I share this experience to show that even most seasoned reefers still make errors and mistakes, no one is exempt.
Before I transferred the tank to the larger one, I tested the water and made it snow in the big tank after a dosing goof.
Had used an online calculator to get the proper amount of soda ash solution to raise DKH up to match the other tank. Well I did not select DKH by goof and selected Meq/l thus throwing the amount needed WAY far too high. Being tired and not paying attention I added the absurd amount to the tank and Would Ya know it was snowing inside hahahaha.
Whole tank was just massive flakes and settled on the bottom in a thick crust that took forever to clean and scrape up.
Good news is the tank was just water and not even cycling through the sump or overflows yet.
Another lesson learned even after my many years, check / double check and check again and if that still isnt enough then have fun correcting your mistake and have a laugh about it later as long as no livestock was hurt in the process, this is when it hurts more.
I agree with chipping away at it, You can use things like tweezer ends to scratch at the surface dislodging particles and if you use a chisel ( a clean one ) then just use the weight of the tool and your hand to punch around it carefully.
Yes the glass is stronger than some give credit for and supports the weight within, though while under full water pressure it can and will crack or go bye bye with the right hit on the right spot and easier than one would think.
It can be done safely just go slow and make progress. It will take some time and even after you will be still finding smaller rocks of sand that will drive you nuts for a while that you will have to remove or grind back down.
